Carrier-signal detector



De- 1, 1942- N. P. CASE CARRIER-SI-GNAL DETECTOR Filed Feb. 25

aoungoosg IUHUN-od ATTORNEY Patented Dec. l, 1942 UNETED STTESVENT OFFICE CARRIER- SIGNAL DETECTOR Nelson P. Case, Great Neck, N. Y., assignor to Hazeltine Corporation,- a corporation of Dela- Ware Application February 25, 1941, Serial No. 380,448

' 12 claims. (c1. 25o- 20) The present invention relates to carrier-signal detectors and, more particularly, to such detectors of the type adapted effectively to increase the amplitude of 4the carrier-frequency component of a modulated-carrier signal applied y thereto with respect to its modulation sidebands.

Present-day radio carrier-signal systems are subject to distortion of the received carrier signal due to the phenomenon of fading. Fading, in general, occurs when the carrier signal is transmitted to the receiver in large part by re-lection from the Kennelly-Heaviside ionized layer surrounding the earth. Generally, the carrierfrequency component and the sideband components simultaneously travel over a number of individual paths including one or more reflections from the ionized layer. One of the most severe cases of distortion occurs when the carrier-frequency component in travelling over its path or paths to the receiver decreases in intensity materially with respect to certain portions of the modulation sidebands, thereby effectively to produce over-modulation of the carrier wave. Such relative changes of intensity of the carrier signal and certain portions of its modulation sidebands are commonly referred to as selective fading.

It has been proposed in accordance with one prior art arrangement that the eifect of selective fading be reduced by the method of separating at the receiver the carrier-frequency component from its modulation sidebands and thereafter combining with the modulation sidebands, before detection, a carrier wave of relatively high and constant Vintensity derived either from a synchronized lo'cal oscillator or by amplification of the separated carrier-frequency component to a predetermined constant level. An arrangement of this nature has the disadvantage that it is diicult to maintain the proper phase relation between the constant intensity carrier wave and the sidebands With which it is combined, a condition that is essential to distortionless reproduction of the modulation components of the received carrier signal. The arrangement has the additional disadvantage of undue complexity of apparatus and circuit arrangement.

It has been proposed in accordance with another prior art arrangement that the carriervfrequency component be effectively increased in amplitude with respect to its modulation sidebands in each of two carrier-signal channels of a receiver, the outputs of the two channels being rectified and combined to derive the modulation components of the carrier signal` Suchemphasis of the carrier-frequency component is effected by neutralized piezo-electric crystals effectively individually ink series in the carriersignal channels. This prior art arrangement has a disadvantage, aside from the complexity of' the circuit Varrangement necessary to'eiiect the desired result, that the signal channels have a number of tuned circuits individual to each which are diicult to maintain in precise adjustment Iover any appreciable period of time and which consequently tend to introduce distortion into the reproduced signal by virtue of such misadjustment. Furthermore, the use of a piezo-electric crystal in this manner in a superheterodyne type of carrier-signal receiver necessitates some form of automatic frequency control system requiring a frequency detector in' addition to an amplitude-modulation detector.

It is an object of the invention, therefore, yto provide an improved carrier-signal detector Wherein'the amplitude of the carrier-frequency component of the carrier wave is greatly increasedwith respect to its sideband components to reduce th'e effect of selective fading in producf ing distortion of the reproduced signal.

It is a further object of the invention to provide, in a carrier-signal receiver of the superheterodyne type, a combined amplitude and frequency carrier-signal detector having input circuits of extraordinarily high ratio ofv inductive reactance to resistance of the order of ten thousand to one, whereby the amplitude carrier-frequency component is effectively increased with respect to its modulation sidebands and Vfrom which an automatic frequency controhpotential is derived for maintaining the intermediatefrequency carrier signal within a few cycles of the mean frequency to which the intermediate-frequency signal channel of the receiveris tuned.

It is an additional object of the invention to provide a combined carrier-signal detectorand automatic amplification control system in which the carrier-frequency component of a received carrier signal is effectively increased with respect to its modulation sidebands and the magnitude vice coupled effectively in shunt to at least portions of both of the sections to cause each of the network sections to have predetermined frequency-response characteristics of the same type with a maximum response substantially at the nominal carrier frequency of a carrier signal to be applied to the network and a minimum response at an individual one of two frequencies spaced substantially equally on opposite sides of the carrier frequency. The detector includes a pair of rectifier devices individually coupled to the network sections, each of the devices having an individual load impedance across which' there is developed a unidirectional modulation voltage, and an output circuit for utilizing at least one of the developed unidirectional voltages.

In accordance with a preferred form of the invention, a carrier-signal detector of the type described comprises a frequency-selective network including coupled primary and secondary windings and means for deriving voltages equal to the sum and difference of predetermined portions of the voltages -developed in said windings by a carrier signal applied thereto. The detector includes piezo-electric crystal means coupled in shunt to the secondary winding for so modifying the frequency response of the network that each of the developed voltages has maximum amplitude substantially at the nominal carrier frequency of the carrier signal and a minimum amplitude at an individual one of two frequencies spaced substantially equally on opposite sides of the carrier frequency. In this preferred form of the invention, the secondary winding preferably includes two sections and that portion of the primary winding voltage which is combined with the secondary winding voltage to derive the aforesaid sum and difference voltages is of the order of one-tenth of the maximum voltage developed in either section of the secondary winding. A pair of similar inductors are preferably serially connected in circuit With the piezo-electric means and on opposite sides thereof for modifying the frequencyresponse characteristic of the network sections to eliminate the effect thereon of one of the resonant responses of the piezo-electric means. The detector includes means for deriving a unidirectional control potential from the voltage developed across the primary winding by the applied carrier signal, the magnitude of this control potential varying with the intensity both of the applied carrier signal and its modulation sidebands.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

Referring now to the drawing, Fig.. 1 is a circuit diagram, partly schematic, of a complete carrier-signal receiver of the superheterodyne type embodying the invention; and Figs. 2-8, inclusive, are graphs representing frequency-response characteristics of one or more circuit components of the Fig. 1 arrangement and are used as an aid in explaining the operation of the invention.

`Referring now more particularly to Fig. 1 of the drawing, there is represented schematically a complete superheterodyne carrier-signal receiver of a conventional design embodying the present invention in a preferred form. In general, the receiver includes a radio-frequency amplifier I0 having its input circuit connected to an antenna system II, I2 and having its output circuit connected to an oscillator-modulator I3. Connected in cascade with the oscillator-modulator I3, in the order named, are an intermediate-frequency amplifier I4 of one or more stages, an intermedivate-frequency amplifier l5, a combined carriersignal detector, automatic amplification control or A. V. C. supply, and automatic frequency control supply IB, more fully described hereinafter, an audio-frequency amplifier II of one or more stages, and a sound reproducer I8.

The output of the automatic frequency control supply of unit IE is coupled to-the input of a frequency shifter I9, the output of which is coupled, in turn, to the oscillator of unit I3.

An automatic amplification control or A. V. C. circuit is connected between the output of the A. V. C. supply of unit IG and the input circuits of one or more of the tubes of the radio-frequency `amplifier I0, the oscillator-modulator I3, and the intermediate-frequency amplifier I4 in conventional manner.

It will be understood that the various units just described may, with the exception of unit I6, be of a conventional construction and operation, the details of which are well known in the art, rendering detailed description thereof unnecessary. Considering, briefly, the operation of the receiver as a whole and neglecting for the moment th-e operation of unit I6 presently to be described, a desired carrier signal is selected and amplified by the radio-frequency amplifier I0, converted to an intermediate-frequency carrier signal in the oscillator-modulator I3, amplified in intermediate-frequency amplifier I4, and detected by the carrier-signal detector of unit I6, thereby to derive the audio-frequency modulation components. The audio-frequency components are, in turn, amplied. in the audio-frequency amplifier I'I and are reproduced by the sound reproducer I8 in a conventional manner.

The automatic frequency control bias derived from the automatic frequency control supply of unit I6 is effective to control the frequency shifter I-B and thereby the operating frequency of the oscillator of unit I3 to maintain the frequency of the intermediate-frequency carrier signal within a few cycles of the nominal frequency of the pass band of the intermediate-frequency amplier I4.

The automatic amplification control or A. V. C. bias derived from the A. V. C. supply of unit I6 is effective to control the amplification of one or more of the units I0, I3, and I4 to maintain the signal input to the detector of unit I6 within a relatively narrow range for a, Wide range of received signal intensities.

Referring now more particularly to the portion of the system embodying the present invention, there is coupled to the output of the intermediate-frequency amplifier I5 a combined carriersignal detector, automatic amplification control or A. V. C. supply, and automatic frequency control or A. F. C. supply I6. The carrier-signal 'acteristics of the same type.

detector ofV unit I6 comprises a frequency-selective network 20 including two sections and a single piezo-electric crystal device 2| coupled effectively in shunt to at least portions of both of the sections to cause each of the network sections to have predetermined frequency-response char- The frequencyselective network 20 includes a transformer 9 having a primary winding 22 and a center-tapped secondary Winding 23, each half of the second- Aary winding comprising one section of the network 20. Condensers 24, 25 are connected across the windings 22, 23, respectively, to tune them to the frequency f the intermediate-frequency carrier signal. Y

In order to derive two voltages equal to the sum and difference of predetermined portions of the voltages developed in the transformer windings 22, 23 by an intermediate-frequency carrier signal applied thereto, there is provided means coupling the windings to provide two network sections comprising a coupling condenser 26 connected between the high alternating-potential terminal of the primary winding 22 and the center tap of the secondary winding 23. The convoltage developed in the primary winding 22 by the intermediate-frequency carrier signal to each secondary circuit including a half of the secondary winding 23. The portion of the primary winding voltage which is applied by the condenser 26 to the secondary circuit is determined by the value of the condenser 2B which preferably is sufficiently small that the portion of the primary voltage applied through the condenser is of the order of one-tenth the maximum voltage developed in either half of the secondary Winding. While the circuit arrangement of the frequencyselective network thus far described is of the conventional split-phase type, the preferred value of the condenser 26 is materially smaller than that conventionally used.

A pair of similar inductors 21, 23 are serially connected in circuit with the piezo-electric crystal 2| and on opposite sides thereof. A pair of rectifier devices such as diodes 29, 39 are coupled to individual halves of the secondary winding 23,

and thereby to individual sections of the networky 20, by individual load impedances 3|, 32, respectively. A common direct current path, comprising a radio-frequency choke 33, is also provided. Unidirectional modulation voltages are developed across each of the load impedances 3|, 32 and a first pair of output-circuit terminals 34, 35, which are included in an output circuit for utilizing at least one of the unidirectional voltages, are coupled to the load impedance 32 and to the input circuit of the audio-frequency amplifier l1 for applying to the latter the developed modulation voltage. A second output circuit comprising the output terminals 35, 36 is coupled across both of the load impedances 3|, 32 in series and to the input circuit of the frequency shifter I9, thereby to derive and to apply to the frequency shifter a unidirectional automatic frequency control bias.

The automatic amplification control supply of unit I6 comprises a rectifier device 31 anda load impedance 38 therefor coupled across the primary winding 22 through a coupling condenser 39. The output of the automatic amplication control supply is coupled through a filter network comprising series-resistance arms 40, 4| and a shunt-capacitance arm 42 to the input denser 26 is effective to apply a portion of the f circuits of one or morewof the tubes of units I0, I3, and`|4, as previously explained.

Before considering the operation of the circuit just described as a whole, it may be noted that the piezo-electric crystal 2| has a frequency-reactance characteristic represented by the curves of Fig. 2.- Ihe crystal is series-resonant at the frequency f1 and parallel-resonant at the frequency f2. If' it be assumed at this point that the coupling condenser 26 be omitted and the inductors 21, 28 be short-circuited from the circuit of the detector of unit I6 and the rectifier device 30 be replaced by a bilaterally conductive path, the voltage developed across the load impedances 3l, 32 in series, with variations of frequency of the intermediate-frequency carrier signal, would be as represented in Fig. 3. This voltage would have minimum amplitude at the series-resonant frequency f1 and maximum amplitude at the parallel-resonant frequency fz of the crystal. The parallel-resonant frequency of the crystal 2| is chosen equal to the nominal frequency of the pass band of intermediate-frequency amplifier I4.

Now, if the inductors 21 and 28 be connected in series with the crystal 2| across the transformer secondary Winding 23, and without any other changes of the detector circuit from that assumed above, the inductors 21 and 2'8 modify the frequency reactance characteristic of the crystal. It has been found that, by properly proportioning the values of inductors 21, 28 with respect to the reactive constants of the selective network, particularly with respect to the capacitance in shunt to the Vcrystal 2|, which capacitance is comprised primarily by the capacitance of the crystal holder, the unidirectional potential developed across the load impedances 3| and 32 in series can be made to Vvary in amplitude with the frequency 'of the intermediate-frequency carrier signal as represented by the curve of Fig. 4 and to have maximum amplitude substantially at the parallel-resonant frequency ,f2 of the crystal. Thus, the crystal and the inductors 21, 28 together simulate a parallel-resonant highly-selective circuit having a ratio of inductive reactance to resistance of the order of ten thousand to one.

In considering the operation of the detector of unit IB in the complete form shown in the drawing, reference is made to Fig. 5 which is a vector diagram representing the magnitude and phase relationships of the voltages developed in the frequency-selective network 2U by an intermediate-frequency carrier signal applied thereto. When the intermediate-frequency carrier signal has a frequency equal to the mean frequency of the network 20, there are developed across the two halves of the secondary winding 23 individual voltages, represented by the vectors a and @which are displaced in phase 180 degrees with respect to each other and degrees with respect to the voltage developed across the primary winding 22', the vector c representing that portion of the latter voltage which is applied through the condenser 26 to the secondary winding 23. The voltages applied to the rectifier devices 29 and 30 are the vectorial sums of the primary voltagevector c and individual ones ofthe vector voltages a, b and are represented in Fig. 5 by the vectors d and e, respectively. As the frequency of the intermediate-frequency carrier signal deviates from the mean frequency vof the network 20, thevoltages induced in each change inV magnitude While maintaining their relative 180-degree phase displacement but vary in i phase with respect to the voltage c developed across the primary winding 22, the magnitude of the latter voltage remaining substantially constant over a relatively wide range of frequency deviations of the intermediate-frequency carrier signal by virtue of the relatively broadly-selective characteristic of the tuned circuit comprising primary winding k22 and condenser 24. The vectors and g represent, for example, themagnitude and phase of the secondary voltages in the network at a particular value of deviation of the intermediate-frequency carrier signal, the resultants of these voltages and the primary voltage c being represented by the vectors d and e. The circles 7 1. and represent the loci of the ends of the vectors c and b, respectively, with deviations of the intermediate-frequency carrier signal over a wide range of frequency deviation on each side of and including the mean frequency of the network 2|). It has previously been stated that the condenser 26 is sufficiently small that the portion of the primary winding voltage which is applied to each half of the secondary winding 23 is of the order of one-tenth the maximum voltageA developed in either half of the secondary winding. It may be noted at this point that the vectorrc of Fig. 5 has not been drawn to scale with respect to the vectors a. and b since to do so would more confuse than aid in arriving at an understanding of the operation of the invention. It will be evident, therefore, that the resultants of the primary and secondary voltages have minimum amplitudes at the points l: and m on the respective circles h and z', these points corresponding to equal wide deviations of the carrier signal at opposite sides of the mean frequency of network 2U, and that the resultant voltages have larger `thari minimum amplitudes for frequency deviations in excess of these values. Thus, the sum and difference voltages developed by the two sections of the frequencyselective network 2|! have frequency-amplitude characteristics as represented by the brokenline curve A and solid-line curve B of Fig. 6, which are of the same type but oppositely directed with respect to the nominal intermediate carrier frequency. The maximum' amplitudes of curves A and B occur substantially at the nominal carrier frequency of the intermediatefrequency carrier signal. The minimum amplitude of curve A occurs at a frequency f7, which corresponds to the frequency represented by the point 7c on the circle h, of Fig. 5 and the minimum amplitude of curve B occurs at a frequency fs, which corresponds to the point m on the circuit i.

The vector diagram of Fig. 5 in several respects is not representative of the conventional prior art split-phase frequency-responsive network. In the first place, in the conventional prior art networks of this nature the primary voltage vector, corresponding to vector c of Fig. 5, is at least as large as, or larger than, the secondary voltage vectors, corresponding to vectors a and b of Fig. 5. Secondly, the conventional prior art networks include primary and secondary network sections having substantially equal ratios of inductive reactance to resistance, generally of the order of two hundred to one at most, with the result that the changes of the primary and secondary voltage vectors with the frequency of the interrnediate-frequency carrier signal are of the same order of magnitude whereas in applicants arrangement, as pointed out above, the primary voltage vector c, Fig. 5, remains substantially constant in magnitude over the range of frequencies of the intermediate-frequency carrier signal within which the major part of the phase and amplitude changes of the secondary voltage vectors occur.

Thus, the frequency-response characteristic of the network 2|! is so modified by the crystal 2| and inductors 2l, 28 that each of the sum and difference voltages developed by the two network sections has a maximum amplitude substantially at the nominal carrier frequency of the intermediate-frequency carrier signal and a mini.- mum amplitude at an individual one of two frequencies spaced substantially equally on opposite sides of the nominal carrier frequency of the intermediate-frequency carrier signal. The unidirectional voltages developed across the load impedances 3| and 32 have amplitude-frequency characteristics similar to curves A and B of Fig. 6. It is evident from these curves that the unidirectional modulation voltage developed across either of the load impedances 3| or 32 is similar to what would be procured if the intensity of the intermediate-frequency carrier signal were greatly increased with respect to its modulation sidebands. By virtue of this fact, the carrier signal, in practice, seldom becomes over-modulated even though the intensity of the received signal carrier materially decreases with respect to the intensity of its sidebands due to selective fading. Consequently, the effect of selective fading in causing distortion of the reproduced signal is greatly reduced. Therefore, the pair of inductors 2l, 28 which are serially connected in circuit with the piezo-electric means 2|, and on opposite sides thereof, modify the frequencyresponse characteristic of the network sections to eliminate the effect thereon of one of the resonant responses of the piezo-electric means. The inductors are so proportioned with respect to the capacitance in shunt to the piezo-electric means 2| that the network sections have predetermined frequency-response characteristics of the same type with a peak response substantially at the nominal carrier frequency of the carrier signal and a minimum response at an individual one of two frequencies spaced substantially equally on opposite sides of the carrier frequency.

It will thus be seen that the piezo-electric crystal device coupled effectively in shunt to the sections of the network 20 maintains a high degree of response of the network sections to the carrier-frequency component while attenuating the response of the sections to the frequencies comprising the sidebands of the carrier signal applied tothe network. Expressed somewhat diiferently, the piezo-electric crystal means, which is coupled in shunt to the secondary winding 23, so modies the frequency-response of the network that each of the voltages has a maximum amplitude substantially at the nominal carrier frequency of the carrier signal and a minimum amplitude at an individual one of two frequencies spaced substantially equally on opposite sides of the carrier frequency. The piezo-electric crystal 2|, therefore, comprises means having a plurality of resonant responses, which responses include a parallel-resonant response substantially at the nominal carrier frequency of the carrier signal to be applied to the network, for modifying the frequency response of the network 20 as described.

The separation of the series-resonant and parallel-resonant frequencies f1 and f2 of Figs. 2 and 3 is not represented to scale for-.purposes of simplifying the drawing,'the separation of these frequencies in practice being of the order of 200 cycles when the crystal 2| has a parallel-resonant frequency of 456 kilocycles, which corresponds closely to the usual intermediate frequency employed in present-day broadcast carrier-signal receivers. cy separation of the points of minimum amplitude of curves-A and B of Fig. 6 is not represented to scale, but is in practice also very small. It is, therefore, evident that the crystal 2I comprises means for greatly narrowing the frequencyresponse characteristic of the two sections of the network 20. The inductors 2l, 28 comprise means coupled in circuit with the piezoelectric crystal 2| for modifying the frequencyresponse characteristic of the two sections of the network 20 to eliminate the eect thereon of one of the resonant responses of the piezo-electric crystal.

The output circuit comprising the output terminal 36 of the automatic frequency control system is coupled to both of the load impedances 3l, 32 in series, differentially to combine the unidirectional modulation voltages developed thereacross to derive and utilize a unidirectional control potential, the magnitude and polarity of which vary, respectively, with the degree and direction of departure of the intermediateA carrier frequency from the mean resonant frequency of the network sections. The amplitude-frequency characteristic of this derived unidirectional control potential is represented by the curve of Fig. '7. The control potential is applied to the input of the frequency shifter I9, thereby to control the frequency of oscillations generated by the oscillator of unit I3 to maintain the carrier frequency of the intermediate-frequency carrier signal substantially at the mean frequency of the pass band of the intermediatefrequency amplifier I4. Y

The frequency-response characteristic of the tuned primary circuit comprising the primary winding 22 and the condenser 24 is represented by the curve of Fig. 8. Because of the coupling Likewise, the frequenof the tuned secondary circuit comprising they secondary winding 23, the condenser 25, and the the intensity of both the carrier-frequency component of the intermediate-frequency carrier signal and the sideband componentsl thereof. The control bias, therefore, does not vary unduly in magnitude upon the occurrence of selective fading of the received carrier signal. Theunidirectional control bias derived by the automatic amplification control system of unit I6 is utilized to control a characteristic of the signaltranslating channel of units It, I3,k and I4that is, to control the amplification characteristic of these units to maintain the signal input to the detector of unit I6 Within a relatively narrow range for a Wide range ofl received signal intensities.

As illustrative of a specific embodiment of the invention, the following circuit constants are given for an embodiment of the yinvention of the type shown in Fig. l:

Rectifier devices 29 and 30 T ype 6]:[6 v a c u u m tu e Rectifier device 37 Type GHG Load impedances 31 and 32 uicuum Resistorv 1 megohm Condenser 100 micr0mi crofarads Resistors 38, 40 and 41 1 megohm Condensers 24 and 25 50-150 mi- `cromicrofai-ads Condenser 26 Approxi- 1 v mately 2 micromicrofarads Condenser 39 100 micromicrofarads Condenser 42 0.01 microfarad Primary winding 22 1.1lmillihenries Secondary winding 23 1.11 millihenries Mutual inductance 22, 23 0.03 millihenry liadio-frequency choke 33 4.5.millihenries lnductors 27 and 28, each 4.1 millihenries Piezo-electric crystal 21 anti-resonant frequency 456 kilocycles The invention is adapted for use in a singlesideband amplitude-modulated carrierI receiver, the only change in the receiver from that described above being that the band-pass selectors of the intermediate-frequency amplifierV I4 are so tuned that the nominal intermediate carrier frequency normally lies near lone edge of the pass band of the band-pass selectors of unit I4.

In this event, the frequency shifter I9 maintains the frequency of the intermediate-frequency carrier signal at a frequency corresponding substantially to that of one edge of the pass-band characteristic of the band-pass selectors of unit lli. The operation of a carrier-signal receiver of this type is the same as that described above except that only the carrier-frequency component Vand one sideband component oi' the received carrier signal are applied to unit `I 6. When the unit i4 of the carrier-signal translating channel is of the type hereinbefore described as adapted to translate the carrier frequency component and one sideband component of the carrier wave which is applied thereto, it Will be evident that the single piezo-electric crystal device 2l is coupled effectively in shunt to at least portions of both of the network sections for effecting in the network sections maximum response at the nominal carrier frequency of the carrier Wave and anA attenuated response at the frequencies comprising the translated sideband component.

While vthe invention hasbeen described as embodied in an amplitude-modulated carrier-signal receiver, it will be evident that it is equally well suited for use in a frequency-modulated carriersignal receiver. In this event, the output terminal' 34 of unit I6 is connected to the cathode of the rectifier device 29 to form a frequencymodulation detector of conventional form. The frequency shifter I9 maintains the nominal frequency of the frequency-modulated intermediatefrequency carrier signal at the mean frequency of intermediate-frequency amplifier I4. The intermediate-frequency amplifier I4 may include,`

if desired, an amplitude-limiting system of conventional form.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modications as fall within the true spirit and scope of the invention.

W-hat is claimed is:

l. A modulated carrier-signal detector comprising, a frequency-selective network including a transformer having primary and secondary windings and means coupling said windings to provide two network sections, a single piezoelectric crystal device coupled effectively in shunt to at least portions of both of said sections to cause .each of said network sections to have predetermined frequency-response characteristics of the same type withA a maximum response substantially at the nominal carrier frequency of a carrier signal to be applied to said network and a minimum response at an individual one of two frequencies spaced substantially equally on oppositesidesof said carrier frequency, a pair of rectifier devices individually coupled to said sections, each of said devices having an individual load impedance across which` there is developed i a unidirectional Vmodulation voltage, and an output circuit for utilizing at least one of said unidirectional voltages.

2. A modulated carrier-signal detector comprising, a frequency-selective network including coupled primary and secondary windings and means for deriving voltages equal to the sum and difference of predetermined portions of the voltages developed in said windings by a carrier signal applied thereto, piezo-electric crystal means coupled in shunt to said secondary winding for so modifying the frequency response of said network that each of said voltages has a maximum amplitude substantially at the nominal carrier frequency of said carrier signal and a minimum amplitude at an individual one of two frequencies spaced substantially equally on opp-osite sides of said carrier frequency, a pair of rectifier devices individually coupled to the secondary winding of said transformer, each of said devices having an individual load impedance across which there is developed a unidirectional modulation voltage, and an output circuit for utilizing at least one of said unidirectional voltages.

3. A modulated carrier-signal detector comprising, a frequency-selective network including two sections and a single piezo-electric crystal device coupled effectively in shunt to at least portions cf both of said sections, said piezo-electric means having a plurality of resonant responses including a parallel resonant response substantially at the nominal carrier frequency of a carrier signal to be applied to said network, means coupled in circuit with said piezo-electric means for modifying the frequency-response characteristic of said network sections to eliminate the effect thereon of one of the resonant responses of said piezo-electric means, whereby each of said network sections has a predetermined frequencyresponse characteristic of the same type with a maximum response substantially at the nominal carrier frequency of said carrier signal and a minimum response at an individual one of two frequencies spaced substantially equally on opposite sides of said carrier frequency, a pair of rectifier devices individually coupled'to said sections, each of said devices having an individual load impedance across which there is developed a unidirectionall modulation voltage, and an output circuit for utilizing at least one of said unidirectional voltages. 4. A modulated carrier-signal detector comprising, a frequency-selective network including twosections and piezo-electric means coupled in shunt to at least portions thereof, said piezoelectric means having parallel resonance substantially at the nominal carrier frequency of a carrier signal to be applied to said network, a pair of inductors serially connected in circuit with said piezo-electric means and on opposite sides thereof for modifying the frequency-response characteristic of said network sections to eliminate the effect thereon of one of the resonant re-y sponses of said piezo-electric means, whereby each of said sections has a predetermined frequency-response characteristic of the same type with a maximum response substantially at the nominal carrier frequency of said carrier signal and a minimum response at an individual one of two frequencies spaced substantially equally on opposite sides of said carrier frequency, a pairof rectifier devices individually coupled to said sections, each of said devices having an individual load impedance across which there isv developed a unidirectional modulation voltage,

and an output circuit for utilizing at least one of said unidirectional voltages.

5. A modulated carrier-signal detector comprising, a frequency-selective network including two sections and piezo-electric means coupled in shunt to at least portions thereof, said piezoelectric means having parallel resonance substantially at the nominal carrier frequency of a carrier signal to be applied to said network, a pair of similar inductors serially connected in circuit with said piezo-electric means and on opposite sides thereof, said inductors being so proportioned with respect to the capacitance in shunt to said piezo-electric means that said network sections have predetermined frequency-response characteristics of the same type with a peak response substantially at the nominal carrier frequency of Vvsaid carrier signal and a minimum response at an individual one vof two frequencies spacedsubstantially equally on opposite sides of said carrier frequency, a pair of rectifier devicesthereto with the voltage developed in each section of said secondary winding, therebyv to derive two voltages equal to the sum and difference of said combined voltages, piezo-electric crystal means coupled in shunt to said secondary winding for so modifying the frequency response of said network Ythat each of said voltages has maximum amplitude substantially at the nominal carrier frequency of said carrier signal and minimum amplitude at an individual one of two frequencies 'spaced substantially equally on opposite sides of "said carrier frequency, a pair of recti-' er devices individually' coupled to the two sections of said secondary winding, each of said devices having an individual load impedance across which there is developed a unidirectional modulation voltage, and an output circuit for utilizing at least one of said unidirectional voltages.

'7. A modulated carrier-signal detector comprising, a frequency-selective network including a primary winding and a two-section secondary Winding and means for combining a relatively small portion of the voltage developed in said primary winding by a carrier signal applied thereto with the voltage developed in each section of said secondary winding, thereby to derive two voltages equal to the sum and difference of said combined voltages, the said portion of said primary voltage having a magnitude of the order of one-tenth that of the maximum voltage developed in either section of said secondary winding, piezo-electric crystal means coupled in shunt to said secondary winding for so modifying the frequency response of said network that each of said voltages has maximum amplitude substantially at the nominal carrier frequency of said carrier signal and minimum amplitude at an individual one of two frequencies spaced substantially equally on opposite sides of said carrier frequency, a pair of rectifier devices individually coupled to the two sections of said secondary winding, each of said devices having an individual load impedance across which there is developed a unidirectional modulation voltage, and an output circuit for utilizing at least one of said unidirectional voltages.

8. A modulated carrier-signal detector com- -r prisinng, a frequency-selective network including a transformer having primary and secondary windings and means coupling said windings to provide two network sections, a single piezo-electric crystal device coupled effectively in shunt to at least portions of both of said sections, each of said network sections having predetermined frequency-response characteristics of the same type with a maximum response substantially at the nominal carrier frequency of a carrier signal to be applied to said network and a minimum response at an individual one of two frequencies spaced substantially equally on opposite sides of said carrier frequency, a pair of rectifier devices individually coupled to said sections, each of said devices having an individual load impedance across which there is developed a unidirectional modulation voltage, an output circuit coupled to one of said load impedances for utilizing one of said unidirectional voltages, and a second output circuit coupled to both of said load impedances for deriving and for utilizing a unidirectional control potential, the magnitude and polarity of which vary respectively with the degree and direction of departure of the carrier frequency from the mean resonant frequency of said network sections.

9. A modulated carrier-signal detector comprising, a frequency-selective network including primary and secondary windings and means for deriving voltages equal to the sum and difference of predetermined portions of the voltages developed in said windings by a carrier signal applied thereto, piezo-electric crystal means coupled in shunt to said secondary winding for so modifying the frequency response of said network that each of said voltages has maximum ampli-- spaced substantially equally on opposite sides of said carrier frequency, a pair of rectier devices individually' coupled to the secondary winding of said transformer, each of said devices having an individual load impedance across which there is developed a unidirectional modulation voltage, an output circuit for utilizing at least one of said unidirectionalvoltagesmeans for deriving a unidirectional control potential from the voltage developed across said primary winding by said carrier signal, the magnitude of said control potential varying with the intensity of both the carrier-frequency component of said applied carrier signal and its modulation sidebands, and a second output circuit for utilizing said unidirectional control potential.

10. In a modulated carrier-signal receiver of the superheterodyne type including a local oscillator and frequency control system therefor, a carrier-signal translating channel, and an audiofrequency amplifier, a carrier-signal detector comprising, a frequency-selective network including primary and secondary windings and means for deriving voltages equal to the sum and difference of predetermined portions ofA the v0ltages developed in said windings by a carrier signal applied thereto, piezo-electric crystal means coupled in shunt to said secondary windings for so modifying the frequency response of said network that each of said voltages has maximum amplitude substantially at the nominal carrier frequency of said carrier signal and minimum amplitude at an individual one of two frequencies spaced substantially equally on opposite sides of said carrier frequency, a pair of rectifier devices individually coupled to said secondary winding, each of said devices having an individual load impedance across which there is developed a unidirectional modulation voltag'e, means for deriving a unidirectional control potential from the carrier-signal voltage developed across said primary winding and for utilizing said control potential for controlling a characteristic of said signal-translating channel, a first output circuit coupled to one of said load impedances for applying the unidirectional modulation voltage de-V veloped thereacross to said audio-frequency amplier, and a second output circuit coupled to both of said load impedances in series for applying both of said unidirectional modulation voltages to said frequency control system to control the frequency of said local oscillator.

11. A modulated carrier-signal detector comprising, a frequency-selective network including a transformer having primary and secondary windings and means coupling said windings to provide two network sections, a single piezo-electric crystal device coupled effectively in shunt to at least portions of both of said sections for maintaining the response of said network sections to the carrier frequency and for attenuating the response of said sections to frequencies comprising the sidebands of a carrier signal applied to said network, a pair of rectifier devices individually coupled to said sections, each of said devices having an individual load impedance across which there is developed a unidirectional modu lation voltage, and an output circuit for utilizing at least one of said unidirectional voltages.

l2. In a modulated carrier-signal receiver including a carrier-signal translating adapted to translate the carrier-frequency component and one sideband component of a carrier wave applied thereto, a carrier-signal' detector comprising, a frequency-selective network couchannel l pled to said channel and including a transformer having primary and secondary windings and means coupling said windings to provide two network sections, a single piezo-electric crystal device coupled efectively in shunt to at least portions of both of said network sections for eiecting in said network sections maximum response at the nominal carrier frequency of said carrier wave and an attenuated response at the frequencies comprising said translated sideband component, a pair of rectifier devices individually coupled to said sections, each of said devices having an individual load impedance across which there is developed a unidirectional modulation voltage, and an output circuit for utilizing at least one of said unidirectional modulation voltages. y v

NELSON P. CASE. 

